1. Field of the Invention
The present invention relates to a diode pumped laser, and particularly to a frequency multiplied diode pumped solid state laser wherein a non-linear, frequency doubling crystal is located in an inert gas purged cell having precise temperature control at relatively high temperatures.
2. Discussion of the Related Art
Diode pumped solid state lasers are efficient, compact and reliable sources of high beam quality optical radiation. The group of solid state lasers includes most commonly the neodymium laser, and also ruby lasers, but there are many others. Triply ionized neodymium is the active material of neodymium lasers. In a crystal, the neodymium is a substitutional dopant (most commonly for yttrium). Neodymium may also be incorporated into a glassy matrix. Neodymium may further form part of a crystal, such as with neodymium pentaphosphate NdP.sub.5 O.sub.14. The most common host for neodymium is yttrium aluminum garnet (YAG), or Y.sub.3 Al.sub.5 O.sub.12. Other common neodymium hosts include yttrium lithium fluoride (YLF), or YLiF.sub.4 ; gadolinium scandium gallium garnet (GSGG), or Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12 ; yttrium aluminate (YALO or YAP), or YALO.sub.3 ; and yttrium vanadate (YVO), or YVO.sub.4. Neodymium may also be hosted by phosphate and silicate glasses. Some more recently discovered neodymium host materials holding promise in the solid state laser field include gadolinium vanadate (GdVO), or GdVO.sub.4 ; and yttrium vanadate (YVO), or YVO.sub.4. Ytterbium (Yb) is also being doped into such crystals as YAG, YLF and YVO. Gadolinium vanadate crystals may also be doped with thulium (Tm) or thulium-holmium (Tm,Ho), rather than with neodymium. Titanium doped sapphire (Ti:Al.sub.2 O.sub.3) and erbium doped YAG (Er:YAG) are also coming into vogue in the solid state laser field.
Table I shows the principal lasing wavelengths for many common solid state lasers. As may be inferred or is apparent from an inspection of Table I, the principal wavelengths of lasing action for most solid state lasers is in the infrared (IR) spectral range. However, it is desirable to convert solid state lasers to lase yin the ultraviolet (UV) spectral range. This frequency conversion is achieved with high efficiency by means of nonlinear optical conversion using nonlinear optical crystals. These crystals are normally arranged in the laser setup within the laser resonator for CW systems and outside the laser resonator for pulsed systems.
Commonly employed nonlinear conversion processes are harmonic generation, such as second and fourth harmonic generation (SHG and FHG, respectively), and sum frequency generation (SFG). Other techniques include Raman shifting, sum and difference frequency mixing and parametric conversion. Harmonic generators may be packaged with the laser. Other techniques such as the above-mentioned ones are normally done using separate accessories. Many nonlinear optical crystals are available for doubling of the light frequency of solid state lasers thus converting the light into the visible range. However, efficient quadrupling and quintupling of laser radiation present significant challenges due to a very limited selection of nonlinear crystals and a necessity for special operating conditions for efficient and long-lasting operation.
Very few nonlinear crystals are available for nonlinear conversions of solid state laser light below 300 nanometers (nm) due to their transparency, non-linear coefficients and adequate birefringence. Of these, beta barium borate (BBO), or .beta.-BaB.sub.2 O.sub.4, lithium borate (LBO), or LiB.sub.3 O.sub.5, and Cesium Lithium Borate (CLBO), or CsLiB.sub.6 O.sub.10, each allow efficient conversion to the shortest wavelengths. CLBO is especially suitable for short wavelength conversion due to its small walk-off angle, and large angular, spectral and temperature bandwidth, leading to more efficient and robust conversion performance. However, CLBO is quite hygroscopic, or sensitive to moisture present in the atmosphere. This results in refractive index distortion and "fogging" of crystal optical surfaces. It has been shown that temperature annealing of the crystal and operation at elevated temperatures significantly reduces index distortion effects. See Taguchi, A., Miyamoto, A., Mori, Y., Haramura, S., Inoue, T., Nishijima, K., Kagebayashi, Y., Sakai, H., Yap, Y. K., and Sasaki, T., Effects of Moisture on CLBO, OSA Topics, in Advanced Solid State Lasers, C. R. Pollock and W. R. Bosenberg, eds., OSA, Vol. 10 (1997).
Practical implementation of the reduced index distortion operations discussed in Taguchi et al. is however complicated due to the tendency of the crystal to absorb and release significant amounts of moisture upon cooling and heating, respectively. This tendency causes damage to optical surfaces of the crystal when it is operated in an enclosed cell, which in turn is necessary for protection from atmospheric moisture. Cooling and heating cycles are unavoidable due to power interruptions, for example, when the device is being transported. Additionally, the temperature of the crystal must be precisely controlled since the phase matching angle for the nonlinear conversion is dependent on the temperature.